1. Field of the Invention
This invention relates to a method as described in the introduction to claim 1.
2. Description of the Prior Art
In one method of the prior art to control a variable-speed high-power drive system (DE 44 22 275 A1), two three-phase motors are each fed by means of an associated pulse-controlled inverter from a direct-current intermediate circuit, which for its part is charged via a line current inverter from a power supply network. The drive motors have a breakdown torque that must always be greater than the torque to be applied. The drive motors are also designed for constant input power above a specified speed, which is determined by the operating point. To thereby prevent interference factors such as oscillating torques or harmonic losses, in particular in the pulse mode, and thereby reduce the electrical and mechanical load on the drive motors as well as the voltage load on the intermediate circuit inverter, in the speed range below the operating point the intermediate circuit direct-current voltage is reduced below the maximum achievable intermediate circuit voltage, and in the speed range above the operating point, starting from the reduced intermediate circuit voltage, is increased in a speed-dependent transition to the maximum intermediate circuit voltage at the maximum speed of the drive motors. The line current inverters, which can be realized in the form of d.c. choppers or four-quadrant controllers, are realized in the form of four-quadrant controllers if the drive motors in regeneration operation also generate a braking torque for the vehicle being driven. During traction operation, the motor voltages increase at a relatively constant magnetic flux from the speed zero to the operating point in a linear fashion with the speed. The maximum phase voltage is thereby achieved at the operating point. At speeds above the operating point, the motor voltage then remains constant as the flux decreases.
The object of the invention is to provide a method for controlling a variable-speed high-power drive system that includes at least one three-phase motor that is designed for constant input power above a specified speed and that is fed from an intermediate circuit converter with a variable intermediate circuit voltage, whereby the intermediate circuit converter is fed from a power line and whereby the intermediate circuit voltage in the speed range below the specified speed is reduced from its maximum value such that the intermediate circuit voltage is not reduced below a minimum voltage specified by the power supply voltage that occurs during operation, and in the speed range above the specified speed. This control method achieves a further improvement in the energy balance of the drive system.
The invention teaches transitioning the intermediate circuit voltage, in a speed dependent manner, from the reduced intermediate circuit voltage to the maximum intermediate circuit voltage at maximum speed. More specifically, above the specified speed and up to the maximum speed, and in the lower range of the torque to be generated or braked by the drive motor, the intermediate circuit voltage is kept at least approximately at the low level that it has below the specified speed up to the maximum speed.
In the invention, account is taken of the finding that in the speed range above the operating point, i.e., in the range of the full-drive setting of the inverter, at the respective speeds in the lower range of the corresponding drive torque to be applied by the drive motor or braking torque to be produced, the corresponding intermediate circuit voltage does not yet need to be increased to the increased level specified by the prior art. The invention teaches that the intermediate circuit voltage can be represented by an improved characteristics map of the form Ud=f(M,n), wherein the intermediate circuit voltage U is a function of torque M and the required speed n. Accordingly, at low torque requirements, the intermediate circuit voltage can be kept largely constant or increased by only a few percent with respect to the intermediate circuit voltage that is used below the operating point, i.e., in the pulse range of the inverter. Only when the torque requirement increases above the operating point does the intermediate circuit voltage need to be increased to meet the demands placed on the drive system with regard to torque and speed to the value specified by the prior art, as a function of the increased torque requirement. The power loss of the drive system is then always minimal. The operating range in which the components of the intermediate circuit converter are supplied with reduced voltage and therefore have a longer useful life with reduced total power loss is thereby increased for speeds above the operating point.
The magnetic flux in the respective drive motor can also be controlled in an analogous manner, in addition to or also as a function of the method described above. Accordingly, at speeds above the operating point, the magnetic flux in the drive motor decreases steadily toward higher speeds, whereby the initial value corresponds to the largely constant value as indicated in the characteristics map below the operating point. The magnetic flux, always with reference to a selectable speed, is kept approximately constant in the lower range of the torque to be applied or braked by the drive motor, and is increased only when higher torques are required. The result, as for the characteristics field for the intermediate circuit voltage, is a trough-shaped characteristics field that decreases to lower values above the operating point. Overall, the magnetic flux decreases continuously toward higher speeds. The drive motor is thus operated above the operating point, starting from low torque requirements in the lower range with optimally low current values that are increased only when the torque requirement increases toward the respective maximum. The magnetic flux thereby reaches the value specified by the prior art only at higher torque requirements. The reduction of the magnetic flux is accompanied by a reduction to a minimal value of the electrical and eddy current losses, whereby at low torque requirements the magnetic flux may only be reduced to the point where it does not drop below the breakdown torque of the drive motor.
If auxiliaries are included in the drive system in the form of fans, pumps or similar devices that are used to cool the individual components such as the transformer, converter and drive motor, it is appropriate to also include their power consumption in the energy balance and to minimize this power consumption as a function of the operating conditions. For this purpose, the cooling power and thus the power supply can be controlled to a desired value below the maximum cooling value until the achievement of a predetermined, maximum allowable temperature of at least one component or part of a component that has a high thermal time constant in the drive system. Above the specified temperature, on the other hand, the cooling power is increased to the maximum value. The control of the power to the auxiliaries and thus of the cooling of the individual components is a function of the temperature of the respective components and the number of the cooling components that can be controlled independently of one another. If the cooling components cannot be controlled independently of one another, the control is exercised by selection of the maximum value.
In addition or alternatively, it can also be appropriate to control the cooling power proportional to the current power loss that occurs in the drive system on components with a low thermal time constant. This method prevents partial overheating of components or parts of components at points that are impossible or difficult to reach with thermal sensors. A more intense ventilation therefore occurs if there are sudden changes in the load in the drive system.
The energy balance can be further optimized if, below the specified maximum temperature and power loss values in the drive system, the cooling power is increased only if the total energy to be expended for the cooling is less than the sum of the additional electrical power losses that occur in the drive system if cooling is not provided. For this purpose, the current power loss of the individual components is determined from the current operating data and a determination is made, e.g. by comparison with empirically determined values, whether it is more favorable, in terms of the overall energy balance, to actuate the auxiliaries now or only later as the temperature or load of the components of the drive system increases and the heat generated by them increases compared to the cooling power currently required.
To improve the energy balance, the invention also teaches the de-excitation of the magnetic flux in the drive motor when there is no torque requirement, i.e. when a rail vehicle is coasting or stopped. This measure also eliminates the energy required for idle operation. The power required for the operation of the auxiliaries is also reduced by adjusting the cooling conditions to the reduced load requirement. This adjustment can continue to the point where individual auxiliaries can be de-actuated.
A rail vehicle operated according to the method taught by the invention and equipped with the devices necessary for such operation has a minimal-loss drive system in which the auxiliaries can also be included.